This invention relates to an improved shot sleeve having at least one passageway for fluid flow, and more particularly, to a shot sleeve in which the temperature across the width thereof can be equalized by providing at least one passageway through which a fluid, having a controlled temperature and flow rate, passes.
Horizontal die casting machines use a horizontal charging chamber called a shot sleeve and a piston rod assembly to deliver molten metal under pressure to the die cavity. In most die casting machines, the shot sleeve is supported in the area of the die and press platen, leaving more than half its length unsupported. In the process of filling the shot sleeve with molten metal prior to injection, the molten metal charge accumulates on the bottom of the shot sleeve for a period lasting from typically 3 to 8 seconds. The molten metal fill percentage of the shot sleeve is maintained within a range of 30 to 70% by volume. This ratio of shot sleeve fill determines the amount of internal shot sleeve surface exposed to molten metal and thereby the rate of temperature change of this surface.
With unequal heating of the shot sleeve occurring, a steady state temperature gradient is reached between the top and the bottom of the shot sleeve. Since the shot sleeve is unsupported for a major portion of its length, the effect of this temperature gradient is a bowing of the shot sleeve in the longitudinal direction and an ovalizing of the diameter in the circumferential dimension. This distortion of the shot sleeve affects piston life as travel along the shot sleeve is not a straight line and clearance with the bore varies around the circumference requiring more clearance than desired to prevent galling. This additional clearance leads to flash generation at the die end with resultant early wear of the piston and shot sleeve and, in the case of vacuum die casting, entrance of contaminating gas at the fill end affecting metal and casting quality.
There have been suggested several "passive" methods of controlling temperature gradients, such as in U.S. Pat. No. 4,926,926 which discloses a three-layer shot sleeve in which a copper layer is welded onto a steel inner sleeve. A high yield strength outer layer is shrunk fit onto the intermediate copper layer in order to, according to the patent, act as a "mechanical straitjacket". The outer and inner steel layers are welded together at the two ends of the shot sleeve. European Patent Document EP-278,208 also shows an inner/outer shrunk fit bimetallic shot sleeve. Finally, Applicants' co-pending United States Patent Application (Attorney Docket No. 123177) discloses a shot sleeve having a thin steel inner layer surrounded by a thicker copper outer layer for effectively dissipating the heat through the shot sleeve in order to control temperature gradients.
Although some of these "passive" systems are effective, it is still desirable to more precisely control and make uniform the temperature gradients in the shot sleeve. U.S. Pat. No. 5,012,856 discloses providing a coolant conduit spaced above and away from the shot sleeve well area. The patent states that this avoids stress cracking and metal chilling problems.
Despite the existence of the shot sleeve disclosed in U.S. Pat. No. 5,012,856 there still remains a need for an effective and efficient shot sleeve that has means for providing uniformity of the temperatures across the width of the shot sleeve.